One of the challenges with the ESP Permanent Magnet Motor (PMM) has always been starting the PMM due to the difficulties in producing torque at zero frequency and operating it efficiently throughout the entire load range. Another challenge in the PCP PMM is the rapid response to the PCP torque changes and performance efficiency. This paper describes the latest VSD functionalities that have been developed to eliminate the above challenges and enhance the PMM performance in both Artificial Lift applications.
Traditionally, IR compensation based on voltage boost has been used to start the ESP PMM, however it can cause overcurrent and step-up transformer saturation. To overcome these issues, the Submersible Motor Control (SMC) mode of the VSD features user-configurable start functions based on current control to increase the robustness of starting the ESP PMM. In addition, the SMC mode features an energy optimizer function designed for the ESP PMM to increase energy efficiency throughout the entire operating range. The VSD uses Direct Torque Control (DTC) for PCP PMM applications to enable superior control performance and energy efficiency.
The SMC's current control-based start functions, such as the ‘Kick-start’ and the ‘Acceleration assistance’ were successfully tested in numerous real ESP wells with PMM and step-up transformer. These functions apply configurable current boosts at low speeds to the PMM for a set period to build sufficient starting torque and to enable robust acceleration of high loads towards the production speed range in a controlled manner. The tests showed that these start functions were able to robustly start and accelerate the ESP PMM to the production speed reference without the motor being pulled out of synchronous rotation. Once the start functions are finished, the SMC motor control switches to the Energy optimizer mode, which automatically minimizes the motor current by a Maximum-Torque-Per-Ampere (MTPA) principle throughout the operation range. Tests showed that with the Energy optimizer the PMM power factor was improved, and the energy efficiency was clearly increased. Especially with partial loads the energy savings can be remarkable compared to traditional V/F curve-based control methods. The high control performance and efficiency of DTC for PCP PMM was demonstrated in several real PCP wells, i.e. sensor-less observer-based backspin control and dynamic torque and speed responses even at low speeds.